The present invention relates in general to four wheel steerable systems for use on motor vehicles having front and rear steerable wheels and, more particularly, to improved control of rear steerable wheels on such motor vehicles during combined braking and steering events and/or combined accelerating and steering events.
Four wheel steering systems for use on motor vehicles having front and rear steerable wheels are known in the art. Typically, the front steerable wheels are manually controlled by the vehicle operator and the rear steerable wheels are mechanically or electronically controlled based on the position of the front steerable wheels and other vehicle operating parameters including, for example, speed, yaw rate, lateral acceleration, steering torque, and tire operation region, i.e., linear or nonlinear.
One particular four wheel steering control system is disclosed in U.S. Pat. No. 5,402,341, which issued to Liubakka et al., is assigned to the same assignee as the present application and is hereby incorporated by reference. The '341 patent discloses an open loop control strategy which improves vehicle handling in the nonlinear region of the tires. In particular, it decreases unwanted understeer when the tires are near the limit of adhesion. Since most current production motor vehicles are designed with some amount of understeer, the ability to reduce this designed-in understeer for certain operating conditions can provide significant handling improvements.
As newer vehicle operating systems including anti-lock braking systems (ABS), traction control, four wheel steering, and unconventional suspensions, e.g., variable damping rate, semi-active, or active suspensions, become more sophisticated, passenger vehicles can be made to perform closer to the tire limits while maintaining or even increasing the handling safety margins. For example, oversteer induced by locking the rear wheels is reduced by ABS, and oversteer induced by excessive rear wheel spin is reduced by traction control. Therefore, cars with such systems may not need as large an understeer characteristic from the suspension and steering. Vehicle understeer may also be reduced significantly in performance sports cars and racing cars where more neutral handling is desired.
While the four wheel steering strategy of the '341 patent is a noticeable improvement over prior art systems during many vehicle operating conditions, during some vehicle operating conditions its operating characteristics are less than desired since it may remove too much understeer.
Thus, there is a need for an improved four wheel steering strategy which overcomes the shortcomings of the prior art.